The subject matter described herein relates generally to methods and systems for imaging fiber structure in tissue using optical polarization tractography.
Organized fibrous structures exist in many parts of the body. Examples include the skeletal muscle, heart, cartilage, neural tissues, and dental tissues. Normal fibrous structure in these tissues, in particular, its directional/orientation organization, is essential for maintaining physiological function. As an example, the unique helical-orientation architecture in myocardia fibers is essential for the pumping of blood. Moreover, in all such tissues, the fibrous structure changes under diseased and pathological conditions. A clear understanding of changes in fiber organization will help to better understand disease mechanisms and develop more effective therapy.
In order to assess the structure and function of these fibrous tissues, traditional histological sectioning, with its ability to achieve cellular level resolution, has long been the gold standard. However, histological sectioning is destructive, time consuming, labor intensive, and only practical for sampling a few small areas in fixed tissues. Further, histology processing works inherently in a two dimensional (2D) plane. Therefore it captures only the 2D projection of a three-dimensional (3D) object, which varies with the specific projection angle.
Diffusion-tensor based magnetic resonance imaging (DTI) has recently been established for imaging the global 3D fiber orientation, especially in the brain and heart. However, DTI suffers from a spatial resolution largely limited to the submillimeter range, which is insufficient for thin and small tissues such as blood vessel, cartilage, and small animal (such as the rodent models of human diseases) tissues.
Optical coherence tomography (OCT) has also been utilized on a commercial basis, but still has limits on its ability to provide cellular-level, 3D optical images of fiber orientation free of significant distortion at cellular levels. In order to resolve fibers using OCT images, the OCT resolution should be sufficiently high to detect intensity changes around the fibers or fiber bundles. Image processing methods were commonly applied within an evaluation window to determine the primary orientation inside the window. The size of the evaluation window sometimes reached a few hundreds of micrometers. Therefore, the actual spatial resolution can be greatly compromised in computing the fiber orientation by OCT. In addition, the intensity based image processing may be affected by intensity changes unrelated to fiber organization. For example, OCT intensity images may contain “banding” artifacts in birefringent tissues caused by the residual polarization effects in the imaging system. Such non-fiber related structural changes may cause incorrect fiber calculations. This issue becomes more problematic at greater depths where intensity contrast deteriorates substantially.
What is needed, therefore, is a system and method for providing true 3D images of fiber orientation and structure inside a tissue at the microstructural cellular-level of imaging resolution without significant distortion. Such a system and method would provide a large imaging area at a high imaging speed, and can do so for fresh and/or in vivo tissues without the need of labeling.
Further, it would be highly beneficial to have an imaging system and method which enables quantitative feedback and high resolution images. Such information will provide researchers and clinicians enhanced assistance in distinguishing distressed, diseased or damaged fibrous tissue of various types from their healthy tissue counterparts.